On 30 October 2014 13:01, Will Deacon [off-list ref] wrote:

On Wed, Oct 29, 2014 at 05:52:00AM +0000, Wang, Yalin wrote:

quoted

This patch add bitrev.h file to support rbit instruction,
so that we can do bitrev operation by hardware.
Signed-off-by: Yalin Wang <redacted>
---
 arch/arm64/Kconfig              |  1 +
 arch/arm64/include/asm/bitrev.h | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 arch/arm64/include/asm/bitrev.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 9532f8d..b1ec1dd 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -35,6 +35,7 @@ config ARM64
      select HANDLE_DOMAIN_IRQ
      select HARDIRQS_SW_RESEND
      select HAVE_ARCH_AUDITSYSCALL
+     select HAVE_ARCH_BITREVERSE
      select HAVE_ARCH_JUMP_LABEL
      select HAVE_ARCH_KGDB
      select HAVE_ARCH_TRACEHOOK

diff --git a/arch/arm64/include/asm/bitrev.h b/arch/arm64/include/asm/bitrev.h
new file mode 100644
index 0000000..292a5de
--- /dev/null
+++ b/arch/arm64/include/asm/bitrev.h

@@ -0,0 +1,28 @@
+#ifndef __ASM_ARM64_BITREV_H
+#define __ASM_ARM64_BITREV_H
+
+static __always_inline __attribute_const__ u32 __arch_bitrev32(u32 x)
+{
+     if (__builtin_constant_p(x)) {
+             x = (x >> 16) | (x << 16);
+             x = ((x & 0xFF00FF00) >> 8) | ((x & 0x00FF00FF) << 8);
+             x = ((x & 0xF0F0F0F0) >> 4) | ((x & 0x0F0F0F0F) << 4);
+             x = ((x & 0xCCCCCCCC) >> 2) | ((x & 0x33333333) << 2);
+             return ((x & 0xAAAAAAAA) >> 1) | ((x & 0x55555555) << 1);

Shouldn't this part be in the generic code?

quoted

+     }
+     __asm__ ("rbit %w0, %w1" : "=r" (x) : "r" (x));

You can write this more neatly as:

  asm ("rbit %w0, %w0" : "+r" (x));

This forces GCC to use the same register as input and output, which
doesn't necessarily result in the fastest code. (e.g., if the
un-bitrev()'ed value is reused again afterwards).
On the other hand, the original notation does allow GCC to use the
same register, but doesn't force it to, so I prefer the original one.

-- 
Ard.